This application relates to and incorporates by reference Japanese patent application no. 2001-166350, which was filed on Jun. 1, 2001.
This invention relates to a capacitive physical load sensor and a capacitive physical load detection system.
An example of a capacitive physical load detection system having a conventional capacitive physical load sensor will first be described by referring to FIG. 14 through FIG. 18. As shown in FIG. 14, the conventional capacitive pressure detection system 1 includes a capacitive pressure sensor 10 and capacitive detection circuits 64. The capacitive pressure sensor 10 includes a pressure sensitive capacitor 20 with pressure capacitance CX and a reference capacitor 30 with reference capacitance CR. The pressure sensitive capacitor 20 is connected to input 60 of a detection voltage VX. Reference capacitor 30 is connected to input 62 of a reference voltage VR. Pressure sensitive capacitor 20 and reference capacitor 30 are connected to the capacitance detection circuits 64. The capacitance detection circuits 64 are connected to an output 78 of a voltage VOUT.
The capacitive pressure sensor 10 is manufactured by forming a diaphragm on a silicon substrate. More specifically, the capacitive pressure sensor 10 includes a silicon substrate 80, a diaphragm 84, which is formed across a gap 82 from the silicon substrate 80, and a retaining part 86 for the diaphragm 84, which is formed around the diaphragm 84, as shown in FIGS. 16 to 18.
Formed on a top surface of the silicon substrate 80 is a pressure sensitive capacitor lower electrode 22b and reference capacitor lower electrode 32b. The pressure sensitive capacitor lower electrode 22b is connected to a pressure sensitive capacitor lower electrode pad 26b through a pressure sensitive capacitor lower electrode lead 24b (see FIG. 15 and FIG. 16), and the reference capacitor lower electrode 32b is connected to a reference capacitor lower electrode pad 36b through a reference capacitor lower electrode lead 34b (see FIG. 15 and FIG. 16). The surface of the silicon substrate 80 is covered by a substrate protective layer 88 (see FIG. 16 through FIG. 18).
The diaphragm 84 includes a semiconductor film 92, which consists of a poly silicon film, and a protective film 96, which consists of a silicon nitride film. A pressure sensitive capacitor upper electrode 22a and a reference capacitor upper electrode 32a are formed on top of the semiconductor film 92. The pressure sensitive capacitor upper electrode 22a is connected to a pressure sensitive capacitor upper electrode pad 26a through a pressure sensitive capacitor upper electrode lead 24a (see FIG. 15 and FIG. 17), and the reference capacitor upper electrode 32a is connected to a reference capacitor upper electrode pad 36a through a reference capacitor upper electrode lead 34a (see FIG. 15 and FIG. 17).
A pressure capacitor 20 shown in FIG. 14 includes the pressure sensitive capacitor upper electrode 22a and the pressure sensitive capacitor lower electrode 22b shown in FIG. 16 through FIG. 18. The reference capacitor 30 shown in FIG. 13 includes the reference capacitor upper electrode 32a and reference capacitor lower electrode 32b shown in FIGS. 16 to 18.
When pressure is applied to the diaphragm 84, the gap 82 acts as a pressure reference chamber that is sealed in a vacuum, and the diaphragm 84 stretches and changes shape in proportion to the applied pressure, as shown in FIGS. 16 to 18. When the shape of the diaphragm 84 changes, the distance between the upper electrode 22a and the lower electrode 22b changes. When the distance between the two electrodes changes, the capacitance between the two electrodes also changes. The circuits shown in FIG. 14 detect a difference between a change in the pressure sensitive capacitance CX of the pressure sensitive capacitor 20 and the reference capacitance CR of the reference capacitor 30 and convert the results into an output voltage VOUT using the capacitance detection circuits 64 in order to detect the magnitude of the pressure being applied on the diaphragm 84.
The reference capacitor 30 makes up for changes in capacitance due to changes in temperature in the environment in which the sensor 10 is placed. As a result, the output voltage VOUT of the sensor 10 is independent of temperature and dependent only on pressure.
In the conventional capacitive pressure sensor 1, which was described above, the output voltage VOUT is proportional to the applied pressure, until the applied pressure reaches a value PA, as shown in a graph in FIG. 19. Once the applied pressure reaches the value PA, the diaphragm 84, shown in FIG. 16 through FIG. 18, comes into contact with the silicon substrate 80, starting at the center, where the diaphragm 84 deforms the most. Beyond this point, the output voltage VOUT gradually becomes saturated and is no longer proportional to the applied pressure. When the applied pressure reaches a value PB, the center part of the diaphragm 84 comes into complete contact with the silicon substrate 80. As a result, the output voltage VOUT is completely saturated with respect to the applied pressure and can no longer represent the applied pressure.
When the diaphragm 84 is thicker, or the diameter of the diaphragm 84 is smaller, the shape of the diaphragm 84 would not change as much with respect to the applied pressure, and it would be possible detect a wider range of pressure levels. However, when the diaphragm 84 is thicker, or the diameter of the diaphragm 84 is smaller, sensor sensitivity suffers. That is, the resolution in detectable pressure is smaller.
An ideal pressure sensor is able to detect a wide range of physical loads (pressure, acceleration, vibration, sound pressure) and offer a high level of sensitivity to detect minute changes in the physical loads across their entire ranges. However, it is difficult to produce such a sensor. On the other hand, a normal application for a capacitive pressure sensor would require a measurement range over which the measurement results must be highly precise, as well as a range over which lower sensitivity is acceptable. In many cases, a lower detectible resolution would be acceptable when the magnitude of the physical load to be measured is large.
Therefore, it is the goal of this invention to provide a capacitive pressure sensor capable of both detecting small changes in pressure across a range over which a high sensitivity is required and of detecting a wide range of pressure levels across a range over which high sensitivity is not required.
This invention is essentially a capacitive physical load sensor including a substrate having a fixed electrode and a diaphragm having a movable electrode. The diaphragm is located across a gap from the substrate. A retaining part for the diaphragm is formed around the diaphragm a protruding part extends from a surface of the substrate or from a surface of the diaphragm into the gap.
The protruding part may be one of a plurality of protruding parts, and surfaces of the protruding parts support the diaphragm when certain physical loads are applied to the diaphragm, respectively.
In a further aspect, the invention may include a correction circuit for correcting a load detection value outputted by the diaphragm, so that the sensor correction circuit issues an output value that changes in a manner that is substantially proportional to changes in the physical load applied to the diaphragm.